1. Field of the Invention
The present invention relates generally to high refractive index materials, and processes for synthesizing them, and applications for using them as optical bulk materials or high index hard coats.
2. The Prior Art
For ophthalmic lenses, plastic materials represent a safer, thinner and lightweight alternative. Such plastic ophthalmic lenses frequently have a surface coating to provide scratch resistance or to impart functional optical features such as tints or anti-reflective surfaces.
Silane based matrices can be used for both coatings and bulk materials. These silanes have reasonably good mechanical properties, but suffer from relatively low refractive index (RI) values, between 1.42 and 1.55. As the demand for thinner and lighter lenses increases, there is a greater need for materials having a higher index of refraction and better mechanical properties. The eye lens industry is focusing on producing high index lenses (refractive index, about 1.6-1.7), which require correspondingly high refractive index (1.63-1.68) coatings. The refractive index of presently available organic coatings is about 1.5, making them unsuitable for high index lenses. Therefore, there is an immediate need for optical grade coatings that have high refractive index.
Plastic lens usually have refractive index as high as 1.67, 1.74 or even 1.80. Conventional coatings usually have a low refractive index of about 1.50. The large difference between the lens substrate refractive index and the coating refractive index causes unsightly fringes. Therefore, it would be desirable to have higher index coatings, and hybrid coatings with correspondingly improved mechanical properties.
One prior hybrid coating is disclosed in US Published Application 2005/0123771. An epoxy silane is hydrolyzed and combined with colloidal silica and an aluminum compound, like an aluminum chelate. The composition has application as an abrasion resistant coating and is useful when applied in conjunction with nonreflective coating layers. Another prior art example is described in US Published Application 2003/165698.
An example of prior art bulk materials is the class of organic polymers. Organic polymers based on thiols and thio-ethers provide a high refractive index (up to 1.70) but are purely organic and not hybrid organic-inorganic. Mechanical properties can be improved by introducing inorganic nanoparticles, however the resulting material is usually hazy due to nanoparticles aggregation.
Hybrid materials, such as transparent hybrid bulk materials are known to be made from silanes leading to an inorganic network. In that case the refractive index is low. The refractive index can be increased slightly by introducing metal alkoxides. However in this case huge differences in kinetics lead to precipitation of the metal alkoxide, limiting the per cent of metal alkoxide content or leading to hazy materials. A prior bulk material is disclosed in U.S. Pat. No. 6,624,237 by hydrolyzing an organo-silicon monomer. The monomer may be combined with an epoxy silane or photochromic compound before hydrolyzing. Another prior art example is described in WO94/25406, which discusses the sol-gel process.
An example of prior art silane coatings is glycidoxypropyltrimethoxysilane, which is referred to by the commercial name Glymo. Glymo is a precursor that is currently used for abrasion resistant coatings in the ophthalmic industry. A high crosslinking rate is achieved, but its refractive index is limited to 1.51. Higher refractive index coatings are obtained by adding high refractive index nanoparticles, such as TiO2 or ZrO2. Such coatings are limited in refractive index due to the low RI of the Glymo. When the content of high RI nanoparticles is increased the coatings becomes brittle and mechanical performance is reduced.
Accordingly, it is an object of the present invention to provide a new class of materials, and their applications as high refractive index bulk materials and coatings.